=Elevated batteries= require much more labor for their construction and for obtaining cover for the men and material than the sunken batteries. They are therefore used only when the target has to be seen and the gun has to be raised for this purpose, or when owing to the presence of rock or water in the soil, or the liability of the site to be flooded it is impracticable to sink the platforms below the surface. As a rule they can be constructed only when covered by a screen either natural or artificial, and then with earth carried in wheelbarrows, sand-bags, etc., etc. =Sunken Batteries.=--When constructed under cover of a screen the depth of the terre-plein of a sunken battery may be limited by the presence of rock or water in the soil, the character of the guns and carriages, and the time available for the work. In a hasty construction the depth of the terre-plein is usually limited to from 3 to 4 feet, which can be dug out in a short time. When more time is available the gun platforms may be put at 5 to 6 feet below the surface and the other parts of the terre-plein may be sunk still lower. This gives but little height of parapet, and the extra earth may be used for giving additional thickness of cover to the splinter-proofs under the traverses and flanks, and also to the magazines. A great variety of plans and profiles may be adopted for batteries of this class, the details of which need not be given, since they will be modifications of those described in Field Fortifications and Permanent Fortifications, and of the exposed battery to be next described. As they are built under cover of screens and are not subject to fire during construction, work upon them may be continuous and by day as well as by night. =46.= =Screens.=--The _natural screens_ used for cover are elevations, woods, hedges, existing buildings, walls, etc., etc. _Artificial screens_ may be made by setting out bushes to imitate hedges or adopting similar devices, which, however, will usually fail to deceive an active enemy. A trench with the earth thrown to the front, forming a glacis-shaped parapet, will, however, generally be effective. It must be made of such length that the enemy cannot know the exact position of the battery, and of such height and thickness that he cannot afford to expend enough ammunition to breach it. This affords not only concealment during construction, but also a remarkably efficient cover to the battery against hostile fire. Screens, natural or artificial, should be from 50 to 100 yards in front of the batteries, so that the enemy’s aim may not be corrected by seeing the points struck by his shells. Unless the screen is of material which will break up into injurious splinters under hostile fire, only enough should be removed before opening fire to unmask the target of each gun, leaving the remainder for concealing the points struck by shells, even if it affords no cover against their penetration.